This application is based upon and claims the benefit of priority from the prior Japanese Patent Application Nos. 2001-62175, filed on Mar. 6, 2001 and 2001-62176, filed on Mar. 6, 2001; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor laser used for a light source of optical communication, optical transmission technology and optical information recording.
2. Description of the Related Art
Recently semiconductor lasers are widely put to practical use as a light source in the field of optical communication, optical transmission and optical information recording, etc., because of coherency of emitted light, of competence for high speed operation or of very small size. The semiconductor laser is mounted on a metallic member such as a lead frame or a metal block so that it can emit a stimulated light thanks to a current flowing from outside thereof and can ensure a passage for heat-radiation because the light intensity varies sensitively in proportion to heat change. To mitigate the difference of thermal expansion coefficient between the metal and the semiconductor material constituting the semiconductor laser, the semiconductor laser is mounted on the metallic member after it is mounted on a substrate material called xe2x80x98sub-mountxe2x80x99 of such as silicon (Si) or aluminum nitride (AlN). Besides, the semiconductor laser is realized by a medium with an amplification rate larger than 1 intervening in a resonator comprised of a plurality of reflection mirrors. An edge emission type semiconductor laser, which utilizes cleaved surfaces of a crystal as the reflection mirrors for the resonator and can take a long distance in the amplifying media through which the light passes, has been mainly developed. Although a surface emission type laser, which emits a radiation in the direction normal to the substrate of high reflection mirror made of such as a semiconductor or a multi-layered dielectric material, is partially put to practical use, there are many problems that the technology is not yet sufficient but mostly still at development stage with regard to a certain material, in other words, to a certain emitted radiation wavelength. Therefore, almost all the lasers utilized in various application products are edge emission type.
However, following problems arise when the edge emission type semiconductor laser is mounted on the sub-mount. A light beam emitted out of the edge diffracts largely and diffuses, because active region of the semiconductor laser is formed as a waveguide structure whose cross section is very small so as to improve the efficiency of amplification, preventing the light from escaping out of the amplification region and therefore causing a loss. In general, because a thin region with a thickness of the degrees of the wavelength can be formed in the direction perpendicular to the component substrate by means of, for example, crystal growth technology, confinement of the light to the region of the degrees of the wavelength is carried out. On the other hand, a confining region is formed by a flat structure in relation to the parallel direction thereto, so that it is difficult to confine the light within the degrees of the wavelength. In addition to the above reason, the light is confined in a region wider than the wavelength in order to prevent the component resistance from increasing. Therefore, diffraction angle expands largely in the perpendicular direction as compared with the parallel direction. For example, the expanding angle of the light beam, i.e. the direction in which the light intensity becomes 1/e2 of the light intensity on the optical axis, is approximately 10 degrees to the optical axis in the parallel direction, whereas the angle is approximately 30 degrees in the perpendicular direction. When the component is mounted on the flat sub-mount, the light beam reaches the mounted surface in the vicinity of the component (for example, at 200 micrometers for the light emitting portion positioned at 100 micrometers above the mounted surface), resulting in reflection, scattering, absorption, etc., which causes deformation of the light beam due to occurrence of so-called xe2x80x98an eclipsexe2x80x99, in a part of the light beam. The phenomenon has a bad influence on connecting it to an optical pickup or an optical fiber utilizing a light beam. Consequently, when the component is mounted, it is necessary to adopt a means such as a structure where the laser component is mounted in the vicinity of the edge of the sub-mount so as to prevent the light from being eclipsed. Therefore, the relation of the position of the semiconductor laser to the position of the mounted surface should be limited, so that flexibility of the mount gets worse.
One method to solve the problem mentioned above is proposed as follows (e.g. Japanese unexamined patent disclosure No. Hei05-315700): The structure of the proposal is that the semiconductor laser component is mounted on a silicon substrate as the sub-mount, and the light beam outputs upward above the substrate by reflecting the beam on a oblique wall surface standing on the substrate. The output light according to this structure is reflected upward in the vicinity of the semiconductor component before it diffuses in a large scale, so that the output light can be taken out with the beam form being practically kept because the eclipse at the mounted surface is small without taking particularly the relation of the position to the mounted surface in consideration.
Meanwhile, the semiconductor laser varies sensitively the light output thereof in accordance with change of the circumambient temperature. Consequently, it is desirable that both the semiconductor laser and the mount substrate should be mounted together on a component being able to control the temperature, for example Peltier device. However because the mount substrate and the sub-mount even have some heat capacitance, though it is small, a method by which feedback control for the driving current circuit of the laser component is carried out by monitoring the actual output light is adopted, if accurate light output control is required. This is called xe2x80x98automatic power control (APC)xe2x80x99.
Because the edge emission type semiconductor laser has both end surfaces formed by e.g. cleaved surfaces working as the resonator mirrors, the output lights are symmetrically emitted in both front and rear directions as far as reflectance of the edges is specially not controlled. Although the aforementioned APC can be constructed by monitoring the rear-side-emitted light with a photo detector, utilization efficiency of the light decreases because the monitored light does not contribute to the signal source. Therefore, a method to improve the utilization efficiency of the light as much as possible by means of raising the reflectance of the rear-end surface with a multi-layered dielectric film, etc., is carried out for a system where raising an output or an efficiency thereof is required. In such a case, the emitted light out of the rear side, which can be used for the monitor, decreases, and consequently the SN ratio becomes too low to carry out the accurate APC. For reasons of the above, it is necessary to monitor a part of the front-side output light (signal light). Such control system is called xe2x80x98front APC (hereinafter denoted by FAPC)xe2x80x99.
FIG. 7 shows an example of the structure where the semiconductor laser is used for the FAPC system. The aforementioned Japanese unexamined patent disclosure No. Hei05-315700 and another Japanese unexamined patent disclosure No.2001-15849 propose a following structure. Namely, as shown in FIG. 7, the structure comprises a semitransparent film formed on a sub-mount 100 and a photo detector for a front monitor, which is formed by p-n junction by means of diffusion process, located behind the film. Mark 101 denotes a semiconductor laser, and a means, such as a half mirror 103, etc. for splitting the output light is positioned facing to an end surface 102 thereof. On the rear surface of the half mirror 103, a photo detector 104 for the monitor is arranged, and an optical input portion 105 is formed on a part thereof. A light 111 emitted from the semiconductor laser 101 is split by the half mirror 103, and a portion of the light becomes an output light 112, which travels toward, for example, an optical disc. A part 113 of the remainder enters the photo detector 104 for the light output monitor so as to output a monitor photocurrent for the APC. Applying the photocurrent to the APC circuit, the output light of the semiconductor laser is controlled. Usually, because the frequency band of the APC circuit is set up lower from dozens to hundreds hertz, the circuit is designed to distinguish slow components of the variation of the output light emitted from the semiconductor laser in order to control them. This is because factor of the variation of the light from the semiconductor laser is mainly variation of the temperature.
On the other hand, considering the application to optical discs, etc., the output light is reflected by the surface of the optical disc, and then enters a signal light detector, etc., after the light is split by a hologram component, etc. inserted on the way. However, because splitting the light with the hologram component utilizes diffraction, 100% of the reflected light is not necessarily diffracted but there are some return lights going back to the light source via the forward path as it is a backward path, due to such as the difference of the double diffractive index of the optical disc medium. To this end, reducing noises caused by the return light of the semiconductor laser is carried out by high frequency superposition technology that continues to give the semiconductor laser a predetermined amount of modulation at a frequency higher than the signal frequency. In FIG. 7, marks 114 and 115 represent the return light and the light incident on the optical input portion 105, which is a part of the return light, respectively. Namely, in the figure, most of the return light 114 is reflected by the half mirror 103, but a part 115 thereof enters the optical input portion 105, then being fed to the APC circuit after it is added to the photocurrent due to the intrinsic emitting light 113 of the semiconductor laser, and resulting in noises. Because the frequency of the signal light included in the return light from the optical disc is usually high such as dozens megahertz to a hundred megahertz, only the average value is detected in relation to high frequency components by the APC circuit having a gain for a low frequency band, and therefore, nothing but minute offset takes place. Since the offset becomes an approximately constant minute value, it can be cancelled by adjustment of the gain of the APC circuit, and then does not matter.
However, because it is necessary that the light pulse for writing should be precisely controlled in the application to random access memory (RAM), utilizing a high speed APC is effective (Proc. SPIE vol.1499, pp.324-329). The high speed APC is thus effective in order to control the intensity of the writing light pulse and to reduce the return light noise.
There is a problem that the monitor noise caused by the aforementioned return light is all amplified as a noise, and in consequence the APC even does not work as well, if the frequency band of the APC circuit becomes equal to or higher than the signal frequency.
As described above in detail, there has been a problem that the return light enters the photo detector for the front monitor and causes some noises on the APC circuit in the semiconductor laser device that emits the light in a direction tilting to the mounted surface, e.g. in the direction perpendicular thereto by means of an oblique surface formed on the sub-mount as a reflection mirror, wherein the reflection mirror is prepared to be semitransparent and the photo detector is positioned on the rear surface thereof.
The present invention is intended to provide a semiconductor laser device wherein any extra light such as a return light does not enter an optical input portion of a photo detector in a front monitor using a semitransparent mirror.
Therefore, an aspect of the invention is a semiconductor laser device comprising:
a semiconductor laser component emitting a laser light; and
an optical control mechanism located in front of the semiconductor laser component and comprising
a semitransparent mirror reflecting the emitted laser light to control the direction of the optical axis of the laser light,
a photo detector united with the semitransparent mirror and receiving a part of light having passed through the semitransparent mirror, and
an optically transparent medium layer intervening between the semitransparent mirror and the photodetector; the optically transparent medium layer having a thickness to make an extra light coming back from the outside to the semitransparent mirror via the optical axis of the reflected laser light deviate from the optical input portion of the photo detector.
This aspect is provided with the transparent layer with a predetermined thickness, intervening between the semitransparent mirror and the photo detector located behind the mirror. The structure can provide an arrangement where the optical axis of the return light does not pass the optical input portion of the photo detector after it passes through the transparent medium layer, and consequently can be controlled by the high speed FAPC system in which all or most of the return lights do not enter the optical input portion, so that the return light noise can be remarkably diminished.